[simcop2387]

Correct, in this case it's the most common type of USB device/driver, the HID driver. Many devices will present themselves as HID Devices initially and even communicate over the HID protocol since it is one of the few USB classes that doesn't require a special driver to be written for windows to recognize it (other classes such as a serial device require at least an INF to be installed in order for windows to load the correct driver).

[cnvogel]

Furthermore, I think, HID is the only standard device class that can work on Low-Speed USB (1.5 Mbit/s) which allows only a control and an interrupt endpoint.

Every other class (including serial ports) will need at least Full-Speed USB (12 MBit) increasing the cost of the USB Rocket Launcher by 10¢ ;-).